1. Field of the Invention
The present invention relates to a color separating optical system for use in a color imaging device.
2. Description of the Related Art
A popular compact digital camera generally adopts a single-panel imaging device using a single image sensor, such as a CCD or a CMOS image sensor, for capturing a full-color image. On the other hand, a high-definition digital camera is provided with a color separating optical system that separates light from the subject into three primary color light components: blue, green and red light components, so that the respective light components will be individually captured as three color-separated images by three image sensors specific for each color. Thus, a high-resolution color image of the subject is obtained from the three color-separated images. These high-definition cameras have been sold in general markets and widely used for professional applications; in photo studios, for broadcastings and in scientific fields, such as astronomical observation through satellite or the like.
A Phillips-type color separating optical system is known as a typical color separating optical system incorporated in a color imaging device. As disclosed for example in U.S. Pat. No. 6,667,656 (corresponding to Japanese Patent Laid-Open Publication No. 2002-365413), the Phillips-type color separating optical system consists of three prisms combined with dichroic films. The first prism has a blue reflecting dichroic film formed on a reflection surface, to reflect the blue light component of the incident light and transmits the red and green light components therethrough. The second prism is spaced with an air gap from the first prism, so that the red and green light components penetrating the first prism will enter the second prism. The second prism has a red reflecting dichroic film formed on a reflection surface, to reflect the red light component and transmit other light components therethrough. The third prism transmits the green light component, which has penetrated the first and second prisms, to emit the green light component from an exit surface of the third prism.
In face to each exit surface of the first to third prisms, an individual image sensor is disposed. A flux of light from the subject enters through an objective lens into the color separating optical system and is separated into blue, green and red light components which are emitted from the exit surfaces of the respective prisms. Thus, the blue, green and red light components of an optical image of the subject are individually captured by the specific image sensors. The Phillips-type color separating optical system is capable of adjusting light path lengths inside the prisms individually for each color. Also a trimming filter may be attached to the exit surface of each prism for the sake of color tone adjustment. These trimming filters have respective spectral characteristics corresponding to the color light components to be emitted from the allocated prisms. Therefore, this type color separating optical system achieves superior color separation and makes optics behind the objective lens compact.
In this type of color separating system, however, when a subject located at the center of the image frame is remarkably brighter than the background, ghosts can be superposed on the subsequent image and the ghosts will remarkably degrade the sharpness of the image. For example, when a spot light source located at the center in a dark background is directly captured, or in a scene where the sun is located at the center for the sake of enhancing the brightness of summer sunshine, or a bright planet in the night sky is located at the center, the ghost phenomenon can occur. The ghosts, which have a tinge of one of the color-separated primary colors, e.g. red, can be superposed on the center subject in the consequent image. Moreover, multiple ghosts can appear at regular intervals within an image frame. Therefore, the ghosts may be an ineligible problem in capturing a scene having a bright subject at the center.
Referring to FIGS. 4 and 5, a comparative example is illustrated for the sake of explaining the above-described ghost phenomenon. In this example, an illumination lamp is captured as a bright center subject through a known color imaging device. This color imaging device includes an objective lens 2, a color separating optical system 6 having first to third prisms 3, 4 and 5, and image sensors 7B, 7R and 7G disposed in face to exit surfaces of the first to third prisms 3 to 5, respectively. The incident light from the subject travels through the objective lens 2 into the first prism 3 along an optical axis P, and falls on a blue reflecting dichroic film formed on a surface 3a of the first prism 3. The blue reflecting dichroic film reflects the blue light component and transmits other light components of the incident light. Blue rays reflected from the blue reflecting dichroic film are totally reflected from an inner surface of an incidence surface of the first prism 3, to exit from an exit surface of the first prism 3. Then the blue rays fall on an imaging surface of the image sensor 7B. Note that when a camera containing the color separating optical system 6 is held at its normal horizontal erecting posture, the color separating optical system 6 will be oriented with its upside in the drawings upward in the field. Accordingly, both the optical axis P of the incident light and the optical axis of the reflected light from the blue reflecting dichroic film will be included in a vertical plane.
An incidence surface of the second prism 4 is opposed to the surface 3a of the first prism 3 with a predetermined air gap S. The second prism 4 has a red reflecting dichroic film formed on a surface 4a. The red reflecting dichroic film reflects the red light component and transmits other light components of the incident light. The red light component reflected from the red reflecting dichroic film is totally reflected from the inner surface of the incidence surface of the second prism 4 and then falls on an imaging surface of the image sensor 7R through an exit surface of the second prism 4. The green light component entering through the incidence surface of the first prism 3 will penetrate through the blue and red reflecting dichroic films and then through the third prism 5, to fall on an imaging surface of the image sensor 7G. The back focus of the objective lens 2 is decided taking account of light path lengths of the respective colors inside the first to third prisms 3 to 5, such that blue, red and green images are formed on the imaging surfaces of the image sensors 7B, 7R and 7G, respectively.
Focusing now on the imaging surface of the image sensor 7R, the red image of the illumination lamp or the center subject is formed in a center area of the imaging surface, and is read as an image signal into the image sensor 7R. Generally, the imaging surface of the image sensor has a micro structure or pattern of indents around individual sensor pixels, regardless of whether it is of CCD type or CMOS type, and the micro structure is made of a relatively-high reflectance material, such as a metal coating. In addition, the micro patterns have a periodic structure corresponding to the sensor pixel array. In the case where a ½-inch or ⅔-inch image sensor has more than 1.5 to 2 mega pixels, the interval between the sensor pixels, which may be called pixel pitch or dot pitch, will be in the order of several microns. Then the micro structure of relatively-high reflectance will behave as a reflective diffraction grating.
As a result, the red rays will be diffractively reflected from the imaging surface of the image sensor 7R toward the exit surface of the second prism 4, and reenter the second prism 4 at diffraction angles that are determined by the wavelengths of the red rays and the pixel pitch. Thereafter the red rays are totally reflected from the incidence surface of the second prism 4 and then fall on the red reflecting dichroic film. Most of the returned red rays are reflected again from the red reflecting dichroic film, and totally reflected from the incidence surface of the second prism 4, and then fall again on the imaging surface of the image sensor 7R through the exit surface of the second prism 4.
The light path of those red rays X being diffractively reflected from the imaging surface of the image sensor 7R and coming back to this imaging surface may be illustrated as shown by dashed lines df1 to df4 in FIG. 5, wherein the second prism 4 is virtually developed relative to the image sensor 7R, as shown by phantom lines, in such a manner that the light paths of the diffracted red rays X after the reentry into the second prism 4 may be shown as straight lines. Generally, the first to third prisms 3 to 5 are made of the same glass material. Assuming that “n” represents a refraction index of the glass material, “λ” a reference wavelength of the red rays, “d” an interval of the indent pattern between the pixels on the imaging surface of the image sensor 7R, and “βm” a diffraction angle of a diffracted beam relative to a perpendicular red light axis Pr to the imaging surface of the image sensor 7R, “m” a natural number (including zero) representative of an order of diffraction, these parameters may be expressed by the following equation:d·n·sin(βm)=m·λ. 
For example, when the refraction index “n” is 1.551, the reference wavelength “λ” of the red rays is 0.61 μm, the interval “d” of the indent pattern on the image sensor 7R is 5 μm, respective diffraction angles “β1”, “β2”, “β3” and “β4” of first to fourth-order diffraction rays df1, df2, df3 and df4 relative to the red light axis Pr (identical to zero-order diffraction ray) will be 4.51°, 9.05°, 13.65° and 18.34°, respectively. Note that “α” in the drawings represents a tilt angle of the red reflection dichroic film to a perpendicular plane to the optical axis P. In the example shown in FIG. 5, the angle “α” is 13.25°.
As seen from FIG. 5, a virtual red light axis Prx representative of the red light axis Pr in the developed view of the second prism 4, of course, intersects with the image sensor 7R at the center of a virtual imaging surface 7Rx in the developed view. In addition, the third-order diffraction rays df3, which have been enhanced by the diffraction, will fall on the center of the imaging surface 7Rx, and form a ghost G3 overlapping on a center subject A1 in an image frame 10, as shown in FIG. 4. Since the indent pattern on the imaging surface of the image sensor 7R forms a matrix grating, the third-order diffraction rays df3 will form multiple ghosts G3 on the right and left sides of the center subject A1 within the image frame 10, and the second-order diffraction rays df2 and the fourth-order diffraction rays df4 will form multiple ghosts G2 and G4 above and below the image frame 10, respectively. This ghost pattern, particularly the ghost G3 formed on the center area of the image frame 10 by the third-order diffraction ray df3, will remarkably degrade the image quality, especially in a scene where a bright main subject is located at the center in a dark background.